1. Field of the Invention
The invention relates generally to a wavelength tunable external resonator laser using an optical deflector driven by an electrical signal and can be applied in the external resonator laser of a Littman-Metcalf mode or a Littrow mode.
2. Description of the Prior Art
An external resonator for tuning a single mode light from a laser diode or other light sources having a predetermined range of bandwidth to select a specific wavelength, includes a Littman-Metcalf mode external resonator and a Littrow mode external resonator. A method by which a specific wavelength is selected using these types of the resonators has been applied to a dye laser technology that is widely researched in the field of a spectroscopy.
FIG. 1 is a structure of a conventional external resonator of a Littman-Metcalf mode.
Referring now to FIG. 1, the external resonator of a Littman-Metcalf mode includes a laser diode 101 having a wide band of wavelength, a first lens 102a for making the beam from the laser diode 101 in parallel, a diffraction grating 104 for diffracting the parallel beam and a reflection mirror 105 for reflecting the diffracted beam. The beam generated from the external resonator laser is reflected against a diffraction grating 104 and focused on an optical fiber 103 via a lens 102b. 
If a beam is generated from the laser diode 101, the beam is converged in parallel by the first lens 102a. Then, the parallel beam is diffracted toward the reflection mirror 105 by means of the diffraction grating 104. At this case, the angle of the reflection mirror 105 toward the diffraction grating 104 is controlled by a mechanical equipment (not shown). Thereby, the reflection mirror 105 reflects specific wavelengths that are vertically incident from the wavelengths incident to the reflection mirror 105, to the diffraction grating 104. The beam reflected by the reflection mirror 105 is diffracted by the diffraction grating 104, so that it returns to the laser diode 101 via the first lens 102a. 
As shown in FIG. 1, if the reflection mirror 105 is positioned at a first angle 109, a first beam 107 of a given wavelength is vertically incident to the reflection mirror 105 and is then reflected toward the diffraction grating 104. Further, if the reflection mirror 105 is positioned at a second angle 110, a second beam 108 having a different wavelength is vertically incident to the reflection mirror 105 and is then reflected toward the diffraction grating 104. As a result, the wavelength of the beam returning to the laser diode 101 is different depending on the angle in which the reflection mirror 105 is positioned. The wavelength is also tuned depending on the angle of the reflection mirror.
As above, the external resonator of the Littman-Metcalf mode controls the angle of the reflection mirror to tune the wavelength. However, the external resonator of the Littrow mode controls the angle of the diffraction grating to tune the wavelength.
FIG. 2 is a structure of a conventional external resonator of a Littrow mode.
Referring now to FIG. 2, the external resonator of the Littrow mode is similar in structure to the external resonator of the Littman-Metcalf mode. Only different is that the external resonator of the Littrow mode does not use the reflection mirror but rotate the diffraction grating 104 to tune the wavelength.
If a beam is generated from the laser diode 101, the beam 201 is in parallel converged by the lens 102. A beam having a specific wavelength from the parallel beams is diffracted depending on the angle 106 of the diffracting grating 104 and is then reflected toward the lens 102. The beam 201 reflected by the diffracting grating 104 returns to the laser diode 101 via the lens 102.
As a result, the wavelength of the beam 201 returning to the laser diode 101 is different depending on the angle in which the diffracting grating 104 is positioned. That is, the wavelength of the beam is tuned depending on the rotation of the diffracting grating 104.
As above, the external resonator tunable laser of the Littman-Metcalf or Littrow mode mechanically rotates the reflection mirror or the diffraction grating and then control the angles of them to select a beam of a specific wavelength. Therefore, as the reflection mirror or the diffraction grating must be mechanically finely rotated, there are problems that the stability of a laser is low, the size of the apparatus is great, the tunable speed is low and the manufacturing cost is high. In other words, the conventional resonator requires a rotation mechanical apparatus having a high accuracy in order to select a specific wavelength and is low in a tunable speed.
Various types of resonators that have been proposed to tune the wavelengths will be now described.
The external resonator laser structure includes two reflection mirrors fixed at both sides of the resonator centering on a laser medium capable of oscillating a plurality of wavelengths so that they can have a rapid variable speed of about 1 ms, and a reflection mirror linearly and in multiple arranged, for varying the length of the resonator by means of PZT.
As the reflection mirror and diffraction grating are simultaneously rotated centering on a given rotation axis located near the laser, the rotation for controlling the diffraction angle and the length of the resonator can be simultaneously controlled. Thus, an external resonator light source can consecutively select a wavelength without hopping a mode.
There is a high-speed wideband wavelength tunable laser system. The laser system includes various tunable components controlled via a microprocessor. The tunable components, being birefringence crystal body representing an electrical optical effect when applied with an electric field, consist of more than two tunable components. At this case, the two tunable components perform a coarse control and a fine control, respectively.
There is a laser resonator including more than two reflection components, positioned at both sides of the resonator, two curve overlapping mirrors, and couple-type reflection mirrors positioned at its output portion. A laser crystal body is installed at a reflection path within the laser resonator. A component for distributing the wavelength such as a prism is positioned at the reflection path within the resonator between one of the overlapping mirrors and the reflection components at its both ends, in order to tune and oscillate at least one wavelength within an expected range of the wavelength. At this time, tuning of the oscillated wavelength is made by a fine rotation of the reflection component.
There is an external resonator structure for tuning the wavelength using an electrical signal without mechanical movement. The external resonator includes two mirrors at its both ends, a crystal body as a laser medium positioned at the center of the mirror, and a crystal body for selecting the wavelength in a piezoelectric unit driven by a RF source as a sound wave input. Therefore, the grating is not moved in the external resonator since the crystal body installed at the piezoelectric unit driven by the RF source.
Also, there is a wavelength tunable laser diode rotates the grating using a stepper motor and controls it using a microprocessor. Further, there is a wavelength tunable laser diode moves the reflection mirror and diffraction grating by means of an actuator using a MEMS technology.
The above-mentioned conventional technologies have advantages in the structure and performance but have some problems. Major problem in the prior arts are as follows: they require mechanical movement and have a narrow wavelength tunable range, and the module size of them could not be miniaturized. In other words, there is a need for a new technology having a spectroscopy the wavelength of which is required to be tuned, a wide tunable range of the wavelength in a WDM optical communication, and a light source having the stability, miniaturization and a rapid tunable speed.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a wavelength tunable external resonator using an optical deflector, which made of a medium capable of controlling the refractive index using an electrical signal in an external resonator of a Littman-Metcalf or Littrow mode is positioned between a reflection mirror and a diffraction grating or between a lens and the diffraction grating in order to control the angle along which a beam travels, so that the wavelength can be consecutively tuned at a rapid speed and the device can be stably driven.
In order to accomplish the above object, an optical deflector according to the present invention, is characterized in that it comprises a p/n junction is formed at a portion of a triangle shape on a substrate of a slab waveguide that is formed using materials capable of forming the slab waveguide such as InP or GaAs, whereby when a beam traveling through the slab waveguide passes through the triangle shape portion of the p/n junction, a refractive angle of the beam is controlled by variations in the density of a carrier in the slab waveguide layer or variations in the refractive index by an photoelectric effect depending on injection of current or application of voltage into/to the p/n junction.
A wavelength tunable external resonator capable of tuning a wavelength using an electrical signal by use of the optical deflector mentioned above according to another embodiment of the present invention, is characterized in that it comprises a light source for emitting beams of various wavelengths; a lens for making the beams in parallel; a diffraction grating for diffracting the parallel beams; a reflection mirror for reflecting an incident beam; and an optical deflector positioned between the diffracting grating and the reflection mirror, for refracting beams among the beam incident from the diffracting grating depending on the electrical signal and then vertically making a beam of a specific wavelength incident to the reflection mirror, whereby the beam vertically reflected by the reflection mirror is focused on the light source.
The optical deflector may be positioned in multiple stages between the diffracting grating and the reflection mirror, so that a beam having a specific wavelength is refracted by at least once wavelength to increase the tunable range.
A wavelength tunable external resonator capable of tuning a wavelength using an electrical signal by use of the optical deflector mentioned above according to another embodiment of the present invention, is characterized in that it comprises a light source for emitting beams of various wavelengths; a lens for making the beams in parallel; a diffraction grating for diffracting the parallel beams; a reflection mirror for reflecting an incident beam; and an optical deflector positioned between the lens and the diffraction grating, for refracting beams among the beam incident from the lens depending on the electrical signal and then making a beam of a specific wavelength directly reflecting from the diffraction grating, whereby the beam directly reflected is focused on the light source.
The optical deflector may be positioned in multiple stages between the lens and the diffracting grating, so that a beam having a specific wavelength is refracted by at least once wavelength to increase the tunable range.
A wavelength tunable external resonator capable of tuning a wavelength using an electrical signal by use of the optical deflector mentioned above according to another embodiment of the present invention, is characterized in that it comprises a diffraction grating of a concave shape, a lens for focusing the parallel light on an optical fiber; a reflection mirror; and a laser diode, the external resonator can be integrated on a semiconductor substrate.